OpenSprinkler Zone Expansion Board Upgraded to v1.1

One of the hallmarks of OpenSprinkler / OpenSprinkler Pi is the capability of expanding the number of zones. While the main controller can only interface with 8 zones, you can expand beyond 8 zones by daisy chaining Zone Expansion boards. Each expansion board adds another 8 zones. Because the zone expansion is implemented using shift registers, there is no hardware limitation on the total number of zones (although there is a software limitation due to the memory space required to store the information and data for each zone). This provides an economic way to implement a large number of zones. On commercial sprinkler controllers, zone expansion is relatively expensive, sometimes requiring you to upgrade to a new controller all together. On OpenSprinkler, if you need more zones, just buy more expansion boards. The software is designed to handle all zones in a consistent user interface.

Previously, the zone expansion board is based on the same PCB as the main controller, and they share the same enclosure. This makes it easy to use a consistent enclosure design for both. However, the zone expansion circuit is actually very simple (just a shift register, a couple of resistors, and eight triacs), so it’s no need to commission a PCB as large as the main controller for something that can be much smaller. So I’ve decided to give the expansion board an upgrade, to version 1.1, which uses a much smaller PCB footprint, and its own dedicated enclosure (based on Serpac WM011).


The new version is just a little over half the size of the original one. So in the same amount of space you can almost fit two expansion boards. This is particularly useful for people who has a limited space to fit a main controller and one or more expansion boards.

In addition, the zone expansion cable has been upgraded to use a polarized connector. Specifically, the connector has a small bump at the top, which matches the notch on the cutout of the enclosure. This way, there is only one possible orientation to insert the cable connector, thus preventing incorrect orientation:


The picture on the left above shows the polarized cutout on the expansion board, and the picture on the right shows a similar cutout on the upcoming injection molded OpenSprinkler enclosure. This can help avoid mistakes when inserting the expansion cable. A special note that this cable has different pin connections with the previous version of expansion boards. So you should NEVER use the polarized cable with previous versions of zone expansion boards, or you may risk damaging your main controller!

Finally, expansion board v1.1 has added a 10K pull-down resistor, which when coupled with OpenSprinkler 2.0, allows the main controller to automatically detect the number of zone expansion boards. This feature is already included in hardware design, but hasn’t been implemented in software yet. The basic principle is that the main controller has a 1.5K pull-up resistor, and each expansion board has a 10K pull-down resistor. When multiple expansion boards are linked together, the pull-down resistors are connected in parallel, thus changing the divided voltage. By using an analog pin (which is internally wired to the voltage division point), the controller can easily calculate how many boards are linked together. A very simple solution!

To conclude, here is a short summary of the new features on OpenSprinkler Zone Expansion Board v1.1:

  • Reduced form factor, and dedicated enclosure.
  • Polarized expansion cable connector.
  • Added pull-down resistor for automatic detection of the number of expansion boards.
  • Besides, the new expansion board retains the on-board PCB holes to fit one MOV per zone, and it works with both OpenSprinkler and OpenSprinkler Pi.

That’s all for the update. This new version is in stock and available for purchase at Rayshobby Shop. Same old price!

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